1. Field of the Invention
The present invention relates to an electric motor, and particularly, to a rotor structure of an electric motor.
2. Description of the Background Art
Generally, a brushless direct current (BLDC) motor is made by removing a brush and a commutator from a direct current motor and by installing an electric rectifying device not to generate mechanical and electrical noise. Therefore, motor having various speed from low to high can be fabricated, and a motor having stable rotation torque and long life span with multiple poles can be made.
FIG. 1 is a longitudinal cross-sectional view showing a conventional BLDC motor, FIG. 2 is a cross-sectional view showing in line IIxe2x80x94II, and FIG. 3 is a brief view showing distribution of magnetic flux density of the motor shown in FIG. 1.
As shown in FIG. 1, the conventional BLDC motor includes a housing body 11 having inner space; a housing 10 having a cover portion 12 for covering an opening portion of the housing body 11 and bearings 13 and 14 installed on center portions thereof; a stator 20 fixedly installed in the housing 10; a rotor 30 rotatably inserted into the stator 20; and a rotary shaft 40 fixedly pressed into the rotor 30 and rotatably inserted into the respective bearings 13, 14 in order to transmit the rotation force of the rotor 30 to outer part.
As shown in FIG. 2, the stator 20 includes a stator core 26 including a plurality of teeth 21 protrudingly formed on an inner circumferential surface thereof, slots 22 formed between the respective teeth 21, and slot opening portions 23 formed between tips 25 which are protrudingly formed on ends of the teeth 21; and a coil 24 wound on the teeth 21 of the stator core 26.
The rotor 30 includes a shaft insertion hole 31, through which the rotary shaft 40 is fixedly press-fitted, on a center portion thereof. In addition, rotor cores 36 on which four magnet insertion slots 32 of xe2x80x98Vxe2x80x99 shape for fixing two permanent magnets which are symmetric for magnetic center respectively are formed as centering around the rotary shaft 40 in a radial direction are laminated and coupled by a rivet through the rivet hole 35 so that two magnetic centers cross each other in right angle on outer side of the shaft insertion hole 31.
The permanent magnets 33 are inserted into ends of the magnet insertion slots 32 in pairs to have equal polarity, and installed to have polarity opposite to that of another permanent magnet 33 inserted into adjacent magnet insertion slot 32.
A recess 34 is formed around the end of the magnet insertion slot 32 in radial direction in order to disperse the magnetic flux density of the permanent magnet 33.
In the conventional BLDC motor having above structure, when electric source is applied sequentially on the coil 24 wound on respective teeth 21 of the stator 20 by an electric source applying circuit (not shown), the respective teeth 21 have alternating polarity of N and S poles, and the rotor 30 is rotated by magnetic flux between the teeth 21 and the permanent magnet 33 of the rotor 30 which is adjacent to the teeth of the stator 20.
At that time, interaction between the two permanent magnets 33 inserted in the magnet insertion slots 32 formed on the rotor 30 to have same polarity on both ends thereof and the teeth adjacent thereto will be described in more detail with reference to FIG. 3.
When the end of the permanent magnet 33 having N polar direction and a tooth 21 having same polar direction, that is, N polar direction, are adjacent to each other on the end of the magnet insertion slot 32, and magnetic forces are repulsed and repulsive force for pushing each other is generated. In addition, when the end of the permanent magnet 33 having N polar direction and a tooth 21 having opposite polar direction, that is, S polar direction, are adjacent to each other, active magnetic flux (a) is formed and magnetic flux density (b) is concentrated to generate attractive force.
The polarity of the teeth 31 can be changed by the electric source flowing on the coil 24 which is wound on the stator core 26, and according to the changes, is the repulsive force or the attractive force is applied with the adjacent permanent magnets 33. In addition, when the repulsive or attractive force is applied toward the tangent direction of the rotor core 36, the rotor 30 can be rotated smoothly.
However, on a tip portion of the tooth 21 having polar direction opposite to that of the permanent magnet 33 on the end of the magnet insertion slot 32, the magnetic flux is concentrated and peak flux (c) is generated, and shock force toward radial direction which interrupts the rotation of the rotor 30 is generated.
In addition, the shock force in radial direction generated due to the magnetic flux increase on the tip 25 of the tooth 21 generates vibration when the rotor 30 is rotated, and accordingly, noise due to the vibration is also generated.
Therefore, an object of the present invention is to provide an electric motor having less vibration and noise by restricting generation of shock force generated around a rotor and teeth of a stator.
To achieve the object of the present invention, as embodied and broadly described herein, there is provided an electric motor comprising: a stator including a plurality of teeth and a coil unit wound on the teeth to form rotating magnetic flux by applied electric source; a rotary shaft; and a rotor rotatably installed in the stator, the rotor including a shaft insertion hole so that the rotary shaft can be inserted therethrough and fixed, wherein the rotor includes a plurality of permanent magnet insertion slots of xe2x80x98Vxe2x80x99 shape formed as centering around the shaft insertion hole in radial direction, and permanent magnets are inserted and fixed into the permanent magnet insertion slots respectively, and a plurality of air gap expanding portions, on which air gaps from the teeth of the stator are expanded, are formed on an outer circumferential surface of the rotor.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.